【doc】Fe（110）面上CO化学吸附状态的吸附历史依赖关系

【doc】Fe（110）面上CO化学吸附状态的吸附历史依赖关系 Fe(110)面上CO化学吸附状态的吸附历 史依赖关系 第9卷第5期 1993年10月 物理化学 ACTAPHYSICO—CHIMICASINICA Ve1.9.No.5 Oct.,1993 ADSOPTIONHISTORYDEPENDENCEOFTHE S1-ATE0FCHEMISORBEDCOONFe(110) Deng#unzhuo'WangHongli' (DalianInstituteofChemicalPhysics,ChineseAcademyof Sciences,Dalian,China116023) Key~rd$iFO(1l9.COchemisorpLion,PES TheehemisorptiouofCOonsinglecrystalsofFehasbeenquiteextensively StUdied.Forinstance,ithasbeenfoundthatthestateoftheehemisorbedCOdepends oTtthee~rfacestrnctureofthesuhstrate,.'.thetemperatureatwhichadsorptiontakes placeiS7.8swellasonthesurfacecoverageoftheadsorbedCO64-Inthiseommu— nicationwepresenttheeffectofadsorptionhistoryoUthestateofehemisorbcdCO onFe(110). TheexperimentswereperformedonaVGESCA—MK?systemwithabasepre- asureofl_5×10一Pa.MgKradiation(1253.BeV)andHeIresonanceline(21.2 eV)wereusedfortheXPSandUPSmeasurementsrespectively.Thebindingenergies intheXPSBrereferredtotheAu4f7,2levelat83.8eV.TheFe(110)samplewa9 mechanicallypolishedtoanopticalfinishwith0.05maluminagritbeforeuse.A proceduredescribedintheliteratureC5]WaSadoptedtoremovethevariousimpurities preseutintheFe(110)sample.Thesurfacecleanlinessofthesampleaftercleaning WasverifiedbytheAugersurfacecompositionalanalysis?TheXPSCl-peakarea afterbackgroundsubtractionWaSusedtomeasuretheamountofCOadsorbedonthe Fe(110)sample.C0withpuritybetterthan99.9WaSusedthroughoutthe experiments. Fig.1givestheXPSspectraofCltand0ltfor(a)COadsorbedat170K, (b)COadsorbedat170Kandsubsequentlyheatedtoroomtemperature,and(c)CO adsoz-bedatroomtemperaturerespectively.ItisclearlyseenthatfortheearleofCO adsorbedatroomtemperature,ashoulderwithalowerbindingenergyoubothClt andOItspectraispresent.Theseshouldersat282.9eVand630.3eVrespectively Received1991一D5—09;revised1999一D6—09. ThisworkwascarriedoutattheStateKeyLaboratoryofSurCaeePhysics.Beliing,199959? China. "Present~ddressIDepartmentofMaterialsEngnesting.Ben—Our~onUniversityofthe Negev,P.0.Boxe5j.Beer—Sheen84105,Israel+ arecharacteristic0fthedis80ci8tedCOasdescribedintheliterature6,73.Whereas no日houIderappearsinthespectrafortheCOadsorbedat170KortheCOadsorbed at170Kfirstandthensubsequentlyheatedtoroomtemperature? ThedifferentstateofthechemisorbedCOinvolvedintheadsorptionat170K withsubsequentheatingtoroomtemperatureandthedirectadsorptionatroomtem— poraturcisfurthersubstantiatedbytheUPSspectratakenforthesesystems-InFig. 2areshowntheHeIUPSspectraofthechemisorbedCOonFe(110)for(a)CO adsorbedat170K,(b)COadsorbedat170Kandsubsequentlyheatedtoroomtem— perature,and(c)COadsorbedatroomtemperaturedirectly.Fortheeaseof(c),the peaksduetodissociatedCOat3.7and5.3eVbelowtheFermilevel[71areclearly seenontbespectruminadditiontothoseascribedtothe4aand1+5amolecular orbitalsofCOrespectivelyasshown. Thus,fortheeaseofCOadsorbedatroomtemperature,partoftheadsorbedCO hasdissociated.BothmolecularlyanddissociativeIyadsorbedCOarepresentonthe surfaceofFe(1l0).ButforthecaseofCOadsorbedat170Kandsubsequently warmeduptoroomtemperature,theohemisorbedstateofCOisexclusivelymolecular innature. BindingEnergy【eV)Bi~dingEnergy【eV Fig.1XP8C】and0,spectrafortheCO/Fe【ll0)Fig.2HeIUPSspectrafortheCO/ systemwithdifferentadsorptionhistoryFe(1IO)systemwithdifferent Ontheleft,C-ispectraadsorptionhistory a)C0adsorbedat170K,b)COadsorbeda)COadsorbedat170K.b)CO at170Kand8ub8equentlyheatedtoroomadsorbedat170Kandsubse. temperature.c)COadsorbsdatroomquentlyheatedtoroomtemlpera. temperature.ture.e)C0adsorbedatrooru Ontheright,OIspectratsmperature a)C0adsorbedatlK.b)C0adsorbedat l70Kandsubsequenflyheatedtoroom temperature.c)C0adsorbedatroom temperature 500五fHUAXUBXUNBAO(A~iaP村.-Ohim.)l990 愁, AnotherfeatureforthedifferenceinthetwoadsorptionsofCOoilFe(D0)with differentadsorptionhist0rvi5thedifferonceintheamountadsorbedasshowninFig. 3.TheuptakeofCOatsaturationa,thelowtemperatureof170Kisfoundtobe highertitanthatatroomtemperature.Thisisexpectedasthemolecularlyadsorbed COwouldnotberestrictedtoamonolaycratthistemperature.Theinterestingpoint , however,whenweraisethetemperaturefrom170Ktoroomtemperaturewith thedesorptionofaportionofthemolecularlyadsorbedCO,theamountofCOadsor- bed0nFe(1l0)finallyobtainedis/lotthesameastheamountofCOadsorbeddi, reetlyatroomtemperature.Theformerishighe~by一2Othanthelatteraaeeenin Fig.3.ThisloweramountofCOadsorbedinthedirectadsorptionatroomtempera— turecertainlyrelatedtothedissociationofCOwhichhasbeenobserved.Ifwe asaun~ethatthedissociationproductoftheatomicspeciesofCoccupiesthesame"long bridgesiteonthesurfaceofFe(1l0)liketheatomicspeciesofO.whichhasbeen dsmoastratedbyGonzalezetdf.[B1.theneachatemacspeeieewouldheexpeotedto hindertheadsorptionof2moleculesofCOinadistortedp(1×2)structureproposed hyE~ley.andsekemetieallyshowni?Fig.4.InthisstructureeachmoleculeofCO isadsorbedterminallyonanFeatom.Thus.thedissociationofICOmoleculewould inactivate4sitesforthemolecularadsorptionofCOonthesurfaceofFe(110).0? thisbasis,thepercentageofdissociatedCOmayhee~timatcdfromFig.3tobeabout 25.ThisisingoodagreementwiththevalueofthefractionofdissociatedCOon Fe(110)at300K(ca?22)determinedbyTDSasreportediatheliteratureE83. IIisthusseenthatthedifferenceinthestate0ftheehemlsorbedCOforthe systemofCO/Fe(I10)withdifferentadsorptionhistoryisexhibitedin(a)thoooour— rBneeofdissociationofCO,approximately25ofthetotalamourLtadsorbed,forthe "."' I,,n-厂l鼻' t f'}J,nrl_j.fhu『I ,——一,_L————一——J—————————— —— 0510152n t/~'(1ll?ill,rj(_lIinsj Fig.3AflsorptlonofCOonFe(110) ~urfacewithdifferentadsorption history 0一@e COFe0C Fig.4Schematicdiagramfortheb[ocklag ofadsDrptionsitesoilFe(110)bythe dissociatedC0 WUL!矿置盯XUBBAO(AotaP^.一^m.)lq日; / 591 j旦2茜×I] caseofdirectadsorption.troomtemperature,and(b)thehigheramountofCOad— sorhcdtotheextentof,20withnodissociationofCOfortheca8eofadsorbing COat170Kfirstandthensubsequentlyheatingtoroomtemperature- TheabsenceofdissociationfortheCOadsorbedatl70Kandsubsequently heatedtoroomtemperaturedeservesfurtherscrutiny.Ithasbeenfoundthatthehigh- lydispersedsingleFeatomsonTiC2(110)canonlyadsorbCOweaklyatroomtam- peraturef.NodissociationofCOwooldbeexpectedundersuchcircumsrances.It hasbeenpostulatedthatforthatsystemthestrongchemisorptionofCOthatleadsto dissociationintheFe/TIO2(110)systemwouldrequirealrlensembleofneighboringFe atomsactingsynergetieally.InourcaseofCOadsorbedat170Kandsubsequently heatedtoroomtemperature.theFe(1l0)surfacewouldbecoveredbyacompletemo— nolaycrofCOat170KwithasecondlayerofmolccularlyadsorbedCOonitstop. Uponwarminguptoroomtemperature,however,themolecularlyphysisorbedCOof thesecondlayerwouldbedesorhed,butthefirstmonolayerofthemolecularlychemi— sorhedCOwouldstillremainonthesurfaceinfullcoverage.Althoughinthisca? neighboringFeatomsarcplentiful,butnoneoghboringFREEatomsofFeareaval— lableonthesurface,Asaresult,dissociationofCOstillwouldnotOCCUr.Thisis camplementarytothefindingsmentionedabove[10],inwhichCOadsorbedoniso— lutedsingleFeatomsisonlyweaklychemisorbed,i.c.,noteapabletodissociate. ItisthussuggestedthatthedissociationofCOontheFe(110)surfacehastwo requirementstobemetl(1)enoughthermalenergytoovercometheenergybarrier fordissociation,thismayheaccomplishedbyraisingthetemperatureofadsorptionor reducingtheheightoftheenergybarrierforCOdissociation,and(2)neighboring FREEatomsofFeattheadsorptionsiteareavailableonthesurface.Forthetwo systemsdealtwithinthiswork,theadsorptionofCOonFe(110)atroomtempera— turehasmetboththerequirements,thereforedissociationofCOoccursattheinltial stageofadsorptionllkethecasereportedforthesystemofCO/Fe(100).Forthe caseofadsorptionofCOat170Kfirstandthensubsequentlyheatedtoroomtemper— atuxe,thedissociationofCOisalthoughenergeticallyfavored,butstructurally unrealizable.Thisappearstobethereasonunderlyingthedifferenceinthebehavior forthetwovasesofCOadsorptionont. hesamesubstrataunderotherwiseidentical conditions,hutwithdifferentadsorptionhistory. AconsiderableamountofdataintheliteratureontheadsorptionofCOonsingle crystalsurfacesatroomtemperatureWaSactuallyobtainedbymakingtheadsorption experimentsatalowtemperature(usuallyevenlowerthan170K)andsubsequently heatingtoroomtemperature.Thismodeofoperationhasbeenusedwiththetacit assumptionthatthestateoftheadsorbedspeciesresultedthereformwouldbethesame asthatobtaJ,neddirectlyfromtheadsorptionatroomtemperature.Itissuggestedthat thisassumptionshouldbeusedwithcautioninthelightofthisstudyonfbecO/Fe 仃f日矿dX置IWgBAO(AeIuPIa--O?妇-)l'9; (110)systemwithdifferentadsorptionhistorypresentedherein. AeknowledgmentlThe~u%horswouldIiketothankLfhong asalstanceinthecourse0fthiswork. REFERENCES IBrodfinO,RhodinTN.BruckerCF,dl口.r,..,1976,59|593 2TextorM,DayID.Ma8oilR.Proc.辟0z0e.'Loildol1,1977JA556,3 3RhodinTN,BruckerCF.f口IComm~m.,1977,2275 lLuJ—P,AIbertMR,BernagekSL.,.i..1989,217-55 5Brod~n0JOafner0,BonzelHP.AppI.P^8.,1977.1333 6KishiK,RobertsMW.,...Farada~.,1975.71l1715 7BroddnOtOafnerO,BonzelHP.rf.占".,1979,82 8OonzalezL.MirandaR,FerrerS.Bmrf.c..1982.119{6l 9ErleyW.,.Vao.cf.~~chnOt.,1981,18|4't2 10DengJZ.WangDZJWeiXMJea1.",.疗...1001.249I213 Fe(110)面~co化学吸附状态的吸附历史依赖关系 趵一5一)至堡琢善弘立 (中国科学院大连化学物理研究所,大难116023) 一摘要Fe(110)面上直接在室温时吸附的c0同先在170K吸附然后升温到室温的c0吸附 有所不同.前者有解离而后者无解离J另外,前者的饱和吸附量比后者低.这可以用前者发 生了解离来解释.根据以上结果,提出了以下的设想,Fe(1l0)面上的c0解离除了需要足够 的热能而外,还需要在面的吸附位上有邻近的其它未被覆盖Fe原子的存在,即需要一定 的表面结构. 关键词:Fe(1l0)CO化学吸附光电予能谱 WULIHUAXUBXUmBAO(Acts.?fm.)l9 一 移欲 7.,? 593